Token passing

The term token passing or English token passing refers to a media access method in computer networks.

Basis of token passing, the token is passed from one station in the network to the neighboring station in a logical ring topology. There are two different forms of implementation of the Token Passing: Token Ring and Token Bus.

For Token Ring neighbor is physically the nearest accessible station, Token bus, it is the logical next available station (realized by the address of the network card ).

Expiration

A free token (consisting of 3 bytes or 24 bits) is constantly being sent from point to point. Want a computer A transmit data to computer C, it waits for the token passes it and then append the token, if it is free, the data packet to, addressed it to computer C and marks the token as busy.

The whole package sends computer A to computer B. Computer B his neighbor realizes that he is not the recipient of the data frame and sends it to its neighbors computer C. Since C is entered as a receiver, it copies the data frame and modifies the token received. Then he sends the frame back to the ring. Since the token is still occupied, no computer can append data. Upon arrival of frames at computer A check A, whether or not the contents match the sent and the reception flag is set.

If this is the case, then the transmission was successful. The data frame is removed and the token is again set free. Even if a transmission is not successful, the transmitter (receipt) definitely has to send a free token after receiving the reception marking. This ensures that after each data transmission is a free token in the ring.

Token passing is used for networks with high load, but also for real-time applications.

• Network architecture